1. Field of the Invention
This invention relates generally to digital processing for pattern or character recognition and more particularly to digital processing for automatic fingerprint identification.
2. Description of the Prior Art
There has been considerable recent interest in the field of automatic fingerprint identification. Generally, this interest has been in an area of optical processing in which a variety of devices and methods have been disclosed. Mainly, these techniques employ match filtering or cross-correlation for direct comparison of the image of the fingerprint to be identified with that of a prerecorded fingerprint image, or for the comparison between the Fourier transform of the image of the fingerprint to be identified and a prerecorded Fourier transform representative of the fingerprint. More sophisticated devices compare certain details of the input fingerprint with prerecorded fingerprint data, for instance, the location of ridge line endings, or the slope or spacing of ridge lines in one region relative to the slope or spacing of the lines in another region. U.S. Pat. No. 3,771,124, issued Nov. 6, 1973 and U.S. Pat. No. 3,771,129, issued Nov. 6, 1973, both in the name of the instant inventor and assigned to the same assignee, are in this general category. The former discloses a coherent type optical processor wherein the fingerprint ridge line orientations are inspected in a plurality of preselected finite areas of the fingerprint by means of a rotating spatial slit filter, disposed in the Fourier transform plane of an optical processor for sequentially transmitting distinct components of the Fourier transform to the image plane of the processor where a plurality of photodetectors are located each corresponding to a discrete sample area. The time delay between a reference orientation of the slit filter and the occurrence of peak light at each detector, which is representative of the ridge orientation in the sample area of the fingerprint, is noted and proportional analog or digital representation thereof is generated for storage and subsequent comparison with similarly obtained signals representative of fingerprints presented for identification. The latter discloses an incoherent optical processor wherein fingerprint ridge line orientations are inspected in a plurality of preselected finite sample areas of the fingerprint by means of a rotating grating which transmits maximum light to a detector, corresponding to a given sample area, when the grating is in spatial alignment with the ridge lines of the related sample area of the fingerprint. The time interval between a reference orientation and the instant of maximum light at each detector determines the ridge line orientation for the detector's sample area. These patents disclose devices for measuring the single most discriminating feature of all fingerprint patterns, the angular orientation of the friction skin ridges as a function of position on the pattern. To perform these measurements both techniques require a rapidly rotating element to perform specialized filtering functions for the determination of ridge orientation.
As opposed to techniques which use optical processing to generate ridge orientation and ridge spacing measurements directly, the present invention provides a digital processing technique by which ridge orientation and ridge spacing can be determined from a binary image of the fingerprint, thus eliminating the need for rapidly rotating elements. It will be apparent to those skilled in the art, that although the invention is described herein with reference to fingerprint analysis for identification, it is also applicable to pattern or character recognition in that line orientation and density in sample areas, of figures to be recognized, can form the basis for identification.